Probably the best feature of RDF is that it forces you to use Uniform Resource Identifiers (URI) instead of private, local identifiers which only make sense in a some context. URIs are long and cumbersome to type, so popular URIs are abbreviated with namespaces prefixes. For instance foaf:Person is expanded to http://xmlns.com/foaf/0.1/Person, once you have defined prefix foaf for namespace http://xmlns.com/foaf/0.1/. In theory URI prefixes in RDF are arbitrary (in contrast to XML where prefixes can matter, in contrast to popular belief). In practice people prefer to agree to one or two known prefixes for common URI namespaces (unless they want to obfuscate human readers of RDF data).

So URI prefixes and namespaces and are vital for handling RDF. However, you still have to define them in almost every file and application. In the end people have copy & paste the same prefix definitions again and again. Fortunately Richard Cyganiak created a registry of popular URI namespaces, called prefix.cc (it’s open source), so people at least know where to copy & paste from. I had enough of copying the same URI prefixes from prefix.cc over and over again, so I created a Perl module that includes snapshots of the prefix.cc database. It includes a simple command line client, that is installed automatically: